AOD-9604 in Canada: Why 176-191 and 177-191 Are Not the Same Label

AOD-9604 is listed in some catalogues as HGH Fragment 176-191 and in others as HGH Fragment 177-191, and it is common to see the three treated as interchangeable names for one product. They are not. AOD-9604 differs from the native fragment by a residue that was added deliberately, and the resulting molecules have different masses. If you are reconciling a certificate against a reference value, or comparing two suppliers listings, that difference is the first thing to establish. This article covers the identity, the numbering, and the one structural feature that makes this compound harder to analyze than its size suggests.

What AOD-9604 actually is

The added tyrosine, and why the numbering disagrees

The parent sequence here is a segment from the C-terminal region of human growth hormone. AOD-9604 is that segment with a tyrosine residue attached at the N-terminus, and that tyrosine is not part of the native protein at that position. It was added as a synthetic modification.

This is the source of the numbering disagreement. A listing that says 177-191 is naming the native segment that forms the body of the molecule. A listing that says 176-191 is, in most cases, loosely accounting for the extra N-terminal residue by extending the range backwards, even though the added residue is a tyrosine rather than whatever occupies position 176 in the native sequence. Both labels get used for AOD-9604 in practice. The problem is that the label 176-191 is also used, correctly, for the actual native fragment without any added residue, which is a different molecule with a different mass.

So a vial labelled HGH Fragment 176-191 might be AOD-9604 under a loose label, or it might be the unmodified native fragment. The product name does not settle it. CAS 221231-10-3 and the molecular mass of 1815.1 do. This is worth checking before comparing purity figures, because a purity figure attached to the wrong identity is not a small discrepancy.

Two sulfurs means a disulfide bond

The molecular formula ends in S2. Those two sulfur atoms are cysteine residues, and in the correctly folded molecule they are joined as a disulfide bond, which closes part of the chain into a loop. This is the structural feature that makes AOD-9604 more demanding analytically than a linear peptide of similar length.

Three things follow from it. First, the oxidized form with the bond intact and the reduced form with two free thiols differ by two mass units. On a molecule of 1815, that is a narrow separation, and a mass determination reported coarsely will not distinguish them. Second, free thiols can form bonds between molecules rather than within them, producing dimers and higher aggregates at roughly twice the target mass. These are a normal impurity class for cysteine-containing peptides and a chromatogram should be read with them in mind. Third, disulfide bonds can form in the wrong pairing when a molecule contains more than one possible arrangement; with only two cysteines the possibilities are limited, which is a point in this compound favour, but it does mean the intramolecular and intermolecular outcomes are the two to watch.

None of this is exotic. It is the ordinary chemistry of cysteine-containing peptides, and it is the reason a method developed for a simple linear sequence is not automatically suitable here.

Handling follows from the chemistry

Free thiols oxidize on exposure to air, and disulfide interchange is accelerated at higher pH. A lyophilized powder in a sealed vial is stable in a way a solution is not, which is a general truth for peptides and a more pointed one for this class. The practical implication is that the interval between reconstitution and use is a variable worth recording, and that a vial which has been opened, left, and returned to is not in the same state as the one described on its certificate.

What the paperwork settles

Batches we have tested and published appear on the certificates of analysis page, indexed so that a vial can be matched to its own document by cap colour and crimp colour. The compound name is not sufficient for that, since successive batches of one product carry different certificates. Not every product and not every batch has a published certificate. Testing, where it has been done, is by HPLC together with purity testing, and the purity guarantee sets out the commitments alongside the claims we do not make. One limit is worth stating plainly for this compound: a purity figure describes the proportion of peptide material that is the target sequence under the method used. It says nothing about disulfide status unless the method was chosen to report on it, and nothing about which of the two competing fragment labels the material was synthesized to.

Where to find it in the range

AOD-9604 5mg is stocked in Canada in a 5 mg presentation. It is catalogued under growth hormone secretagogues for navigational reasons, alongside Sermorelin, Tesamorelin and the CJC-1295 listings, but it is not structurally one of them. Those are releasing-factor analogues built on the GRF sequence. AOD-9604 is a fragment of growth hormone itself with an added residue. Shared shelf space is not shared chemistry, and the two groups have different analytical requirements. Current pricing is on the Canadian price index.

Points to carry away

AOD-9604 is the hGH 177-191 fragment with a tyrosine added at the N-terminus, mass 1815.1, CAS 221231-10-3. The labels 176-191 and 177-191 are both used for it and one of them is also used for a different, unmodified molecule, so verify by CAS and mass rather than by name. The two sulfurs are a disulfide bond, which brings reduced forms two units away and dimers at roughly double mass into the picture. It shares a category with the GRF analogues and shares nothing structural with them.

All compounds referenced here are supplied by ExoLabz for laboratory research use only. They are not for human or veterinary use, are not authorised by Health Canada as a drug or natural health product, and nothing on this page is guidance on preparation, administration or use.